1. Field of the Invention
The present invention relates to a device for purifying the exhaust gases and a control unit therefor, which are provided in an exhaust system of an engine in order to remove pollution components contained in the exhaust gases emitted from a diesel engine and, particularly, to remove particulate matter which is to be reduced in connection with exhaust gas regulations.
2. Description of the Related Art
As an important way to protect the environment, regulations have now been legislated for reducing nitrogen oxides (NOx) and hydrocarbons (HC) which are pollution components contained in the exhaust gases emitted from the engines for vehicles, and a variety of technologies have been vigorously developed to reduce such pollution components. In particular, the regulations concerning the exhaust gases of diesel engines are becoming stringent in recent years, and it is expected that more strict regulations will be legislated in the future. A diesel engine is the one in which the air fed into a cylinder is compressed to reach high temperature and a fuel is injected into the compressed air so as to be burned, and features a higher thermal efficiency than that of the gasoline engines. Therefore, the emission of carbon dioxide (CO2) in the diesel engine is less than the gasoline engine correspondingly, it is strongly demanded to reduce the emission of particulate matter (PM) and Nox in the diesel engine.
The particulate matter is exhausted in the form of particles of carbon and unburned fuel components as a result of incomplete combustion of the fuel injected into the cylinders. In some operating conditions, the diesel engine exhausts the particulate matter in increased amounts due to defective mixing of the air and the injected fuel. In particular, when the so-called EGR is executed to recirculate the exhaust gas being mixed with the air into the cylinders of the diesel engine in order to decrease the NOx, the air is fed in a decreased amount into the cylinder and a maximum temperature of combustion gas decreases causing the particulate matter to be emitted in increased amounts. Thus, there exists a conflicting nature between decreasing the particulate matter and decreasing the NOx.
In order to prevent the emission of the particulate matter, there has been proposed a technology of providing the exhaust system of the diesel engine with a filter called diesel particulate filter (DPF) to trap the particulate matter. The DPF usually comprises a ceramic body such as porous cordierite in which a number of fine passages are formed in the axial direction, the inlets and outlets of the neighboring passages being alternately closed. The exhaust gases of the diesel engine flow toward the downstream passing through the porous ceramic walls between the neighboring passages whereby the particulate matter in the form of fine particles is trapped. Instead of using the porous ceramic body, there is another means to use a nonwoven fabric of a fine texture comprising a heat-resistant fiber such as ceramic fiber. To prevent the emission of particulate matter, further, there has been known a technology of providing the exhaust system with an oxidizing catalytic device. The oxidizing catalytic device has many passages formed in the ceramic substrate and the surface of the passages are coated with a catalyst of a noble metal such as platinum, palladium or rhodium. The exhaust gases flow through the passages in the oxidizing catalytic device whereby the particulate matter in the exhaust gases is combined with oxygen in the exhaust gases from the diesel engine due to the catalytic action, and is converted into CO2 and the like. Such catalyst is often carried on the surfaces of the DPF mentioned above.
The particulate matter trapped on the DPF increases as a result of the repetitive operation of the diesel engine installed on a vehicle. When the particulate matter deposits in large amounts, the filter is choked causing such troubles as increased back pressure of the engine. The large amounts' deposition also causes thermal damage to the DPF due to the combustion of a lot of the particulate matter, which occurs when the temperature of the exhaust gas is elevated during the high-load operation of the engine. To prevent such troubles, the so-called DPF regeneration must be executed to restore the function of the DPF by suitably removing the deposited particulate matter.
As means for regeneration, there has been known a system of burning the particulate matter by heating by using an electric heater or a burner. When the system for burning the particulate matter is employed, however, the DPF must be combined with the electric heater; i.e., the DPF becomes complex and expensive. Besides, the particulate matter cannot be trapped while the deposited particulate matter is being burned, consequently, the system must be such that the exhaust passage is provided with a plurality of DPFs in parallel to alternately execute the trapping and the burning, arousing a problem in that the system becomes bulky.
In view of this problem in recent years, attention has been given to a system of regenerating the DPF by arranging an oxidizing catalyst on the upstream of the exhaust gas of the DPF. The oxidizing catalyst oxidizes the unburned components in the exhaust gas to elevate the temperature of the exhaust gas, and removing the particulate matter trapped by the DPF is carried out continuously while the engine is in operation by the exhaust gas of elevated temperature. Instead of providing the catalyst on the upstream side, there can be further contrived a method of coating the catalyst on the surfaces of the DPE, e.g., carrying the so-called NOx occluding and reducing catalyst on the surfaces of the DPF on the upstream thereof to continuously oxidize and remove the trapped particulate matter by utilizing active oxygen that is generated at the time of occluding and reducing the NOx. The DPF which has the catalyst on the upstream thereof and is regenerated by continuously removing the trapped particulate matter is referred to here as continuously regenerating DPF.
The continuously regenerating DPF removes the particulate matter by the action of the catalyst provided on the upstream thereof and, hence, does not exhibit a sufficient regenerating function when the temperature of the catalyst is not higher than the activating temperature and cannot be continuously regenerated, such as the ordinary catalytic devices does not exhibit a sufficient function in low temperature. A temperature of about 350° C. is necessary for the catalyst to be activated and favorably regenerated. However, during the low-load operation of the diesel engine where the fuel is injected in small amounts, the temperature of the exhaust gas becomes considerably low. If this operating condition continues for extended periods of time, the temperature of the catalyst becomes lower than the activating temperature. Therefore, the particulate matter deposits on the DPF and could cause high back pressure of the engine or melt-damage to the DPF due to the combustion of the particulate matter of large amounts when the temperature of the exhaust gas is elevated. Even in the continuously regenerating DPF, therefore, the particulate matter must be removed by such a method as activating the catalyst by elevating the temperature of the exhaust gas when the particulate matter has deposited in a predetermined amount on the DPF. The above-mentioned regeneration of the DPF is hereinafter referred to as forced regeneration.
The temperature of the exhaust gas of the diesel engine can be elevated by means called post-injection. In the post-injection, an additional fuel is injected into the engine cylinder in the expansion stroke or in the exhaust stroke of the diesel engine, so that the fuel does not burn in the cylinder but burns or be oxidized chiefly in the exhaust pipe and in the catalyst placed therein to elevate the temperature of the exhaust gas. Generally, the fuel is injected after the final stage of the expansion stroke to obtain a favorable effect. The post-injection is such that the additional fuel is fed from a fuel injection nozzle that has been provided already in the cylinder of the diesel engine, and does not require any additional device, which is an advantage. The amount of the post-injection and the number of times thereof may be controlled to adjust the temperature of the exhaust gases to be elevated.
The temperature of the exhaust gas of the engine also elevates if the timing of the ordinary fuel injection of the diesel engine is delayed. The ordinary fuel injection takes place from the end of the compression stroke to the expansion stroke to burn the fuel in the engine cylinder, and delaying the timing of the ordinary fuel injection increases portion of the fuel that does not contribute to producing the torque of the engine, so that the temperature of the exhaust gas rises. A so-called multi-injection is preferred for realizing the delay in the injection timing. The multi-injection is to inject the fuel in a manner of being divided into a plurality of times. In the diesel engine, the controlled delaying of the injection timing can be easily performed by the injection of the fuel being divided into a plurality of times, the fuel that is continuously injected ordinary from the end of the compression stroke to the expansion stroke.
To elevate the temperature of the exhaust gases to maintain catalytic activity of the continuously regenerating DPF, the post-injection or the multi-injection is an effective means, however, this is often not enough to sufficiently elevate the temperature. Therefore, there can be contrived means for promoting the regeneration of the DPF by providing an exhaust throttle valve downstream of the continuously regenerating DPF. The exhaust throttle valve squeezes the exhaust passage by decreasing the opening degree of the valve when the forced regeneration based on the post-injection or the like is executed, to prevent the radiation of heat from the continuously regenerating DPF and, hence, to retain the temperature. Such device for purifying the exhaust gases of diesel engines, i.e., the combination of the continuously regenerating DPF and the exhaust gas throttle valve, has been known as taught in, for example, JP-A-2003-343287. Here, it is noted that if the exhaust gas is squeezed by using the exhaust gas throttle valve, the engine back pressure rises and an increased load is exerted on the engine. Accordingly, the amount of fuel injection further increases and the temperature of the exhaust gas is elevated. The device mentioned above for purifying the exhaust gas of the diesel engine will now be described with reference to FIG. 5.
FIG. 5 schematically illustrates a diesel engine which has a continuously regenerating DPF and an exhaust throttle valve, and drives a vehicle. The air is fed into the cylinders of a diesel engine body 1 through an air cleaner 2 and an intake pipe 3. The fuel is injected into the cylinders from the fuel injection nozzles 4 at the end of the compression stroke, mixed with the compressed air, and burns in the cylinders to produce the power. The exhaust gases after burned are discharged into an exhaust pipe 5 and are partly recirculated into the intake pipe 3 through an EGR passage 6. The recirculation purposes chiefly for preventing the generation of NOx. The amount of the exhaust gases to be recirculated is controlled by the EGR valve 7.
In the exhaust pipe 5, there are arranged a continuously regenerating DPF 8 as well as an exhaust throttle valve 9 on the downstream thereof. The exhaust throttle valve 9 is opened and closed by a fluid pressure actuator that is controlled by an electromagnetic valve 91, and is, usually, maintained fully opened while the diesel engine is in operation. The continuously regenerating DPF 8 includes a DPF 81 having many passages formed in the ceramic body in the axial direction thereof and an oxidizing catalyst 82 arranged on the upstream thereof. The continuously regenerating DPF 8 is further provided with a pressure differential sensor 83 for detecting the pressure differential between the pressure on the upstream of the DPF 81 and the pressure on the downstream thereof, an inlet temperature sensor 84 for detecting the temperature of the exhaust gas on the upstream of the oxidizing catalyst 81, and an outlet temperature sensor 85 for detecting the temperature on the outlet side thereof (inlet side of the DPF 81). Detection signals of these sensors are input to an engine control unit (ECU) 10.
While the diesel engine is in operation, the fuel injected from the fuel injection nozzle 4 burns in the cylinder, and the exhaust gases after the combustion are emitted into the exhaust pipe 5. When the exhaust gases pass through the continuously regenerating DPF 8, the particulate matter contained therein is trapped on the wall surfaces among many passages formed in the DPF 81 in the axial direction thereof, and the exhaust gases from which the particulate matter is removed are discharged to the downstream of the DPF 81. While the diesel engine is in operation, the particulate matter trapped and deposited on the DPF 81 is oxidized and removed upon being bonded with oxygen and the like in the exhaust gases heated at a high temperature by the action of the oxidizing catalyst 82.
However, when the diesel engine is operated carrying a low load for extended periods of time, the temperature of the exhaust gas decreases, the activity of the oxidizing catalyst 82 decreases. Accordingly, the particulate matter is trapped and deposits in increased amounts and pressure differential increases between the pressure on the upstream of the DPF 81 and the pressure on the downstream thereof. When low temperatures are detected by the inlet temperature sensor 84 and the outlet temperature sensor 85 of the oxidizing catalyst 82 and, besides, when the pressure differential detected by the pressure differential sensor 83 exceeds a predetermined value, the ECU 10 produces an instruction signal for effecting the post-injection to elevate the temperature of the exhaust gases, which is for forced regenerating the DPF 81. At the same time, the ECU 10 sends an instruction to the electromagnetic valve 91 to decrease the opening degree of the exhaust throttle valve 9, and executes a control to strongly squeeze the exhaust gas flow.
In the forced regeneration, the fuel fed by the post-injection is oxidized and burns through the exhaust pipe 5 or the oxidizing catalyst 82, and the temperature of the exhaust gases is elevated. Further, the downstream of the continuously regenerating DPF 8 is squeezed by the exhaust throttle valve 9, the high temperature in the continuously regenerating DPF 8 is retained and an increased load is exerted on the engine. Therefore, the oxidizing catalyst 82 is fully activated to promote the regeneration of the DPF 81.